Volumetric energy recovery system with three stage expansion

ABSTRACT

A method for generating mechanical work via a closed-loop Rankine cycle includes heating a working fluid to at least a partial vapor state, generating useful work at a first expansion stage by expanding the working fluid as the working fluid passes through the first expansion stage, generating useful work at a second expansion stage by expanding the working fluid as the working fluid passes through the second expansion stage, generating useful work at a third expansion stage by expanding the working fluid as the working fluid passes through the third expansion stage, and condensing the working fluid to a liquid state.

RELATED APPLICATIONS

This application is a Continuation of PCT/US2014/013393, filed 28 Jan.2014, which claims benefit of U.S. Patent Application Ser. No.61/757,533 filed on 28 Jan. 2013, claims benefit of U.S. PatentApplication Ser. No. 61/810,579 filed on 10 Apr. 2013, and claimsbenefit of U.S. Patent Application Ser. No. 61/816,143 filed on 25 Apr.2013 and which applications are incorporated herein by reference. To theextent appropriate, a claim of priority is made to each of the abovedisclosed applications.

GOVERNMENT LICENSE RIGHTS

This invention was made with government support under Contract No.DE-EE0005650 awarded by the National Energy Technology Laboratory fundedby the Office of Energy Efficiency & Renewable Energy of the UnitedStates Department of Energy. The government has certain rights in theinvention.

TECHNICAL FIELD

The present disclosure relates to a volumetric fluid expander used forpower generation in the Rankine cycle.

BACKGROUND

The Rankine cycle is a power generation cycle that converts thermalenergy to mechanical work. The Rankine cycle is typically used in heatengines, and accomplishes the above conversion by bringing a workingsubstance from a higher temperature state to a lower temperature state.The classical Rankine cycle is the fundamental thermodynamic processunderlying the operation of a steam engine.

In the Rankine cycle a heat “source” generates thermal energy thatbrings the working substance to the higher temperature state. Theworking substance generates work in the “working body” of the enginewhile transferring heat to the colder “sink” until the working substancereaches the lower temperature state. During this process, some of thethermal energy is converted into work by exploiting the properties ofthe working substance. The heat is supplied externally to the workingsubstance in a closed loop, wherein the working substance is a fluidthat has a non-zero heat capacity, which may be either a gas or aliquid, such as water. The efficiency of the Rankine cycle is usuallylimited by the working fluid.

The Rankine cycle typically employs individual subsystems, such as acondenser, a fluid pump, a heat exchanger such as a boiler, and anexpander turbine. The pump is frequently used to pressurize the workingfluid that is received from the condenser as a liquid rather than a gas.Typically, all of the energy is lost in pumping the working fluidthrough the complete cycle, as is most of the energy of vaporization ofthe working fluid in the boiler. This energy is thus lost to the cyclemainly because the condensation that can take place in the turbine islimited to about 10% in order to minimize erosion of the turbine blades,while the vaporization energy is rejected from the cycle through thecondenser. On the other hand, the pumping of the working fluid throughthe cycle as a liquid requires a relatively small fraction of the energyneeded to transport the fluid as compared to compressing the fluid as agas in a compressor.

A variation of the classical Rankine cycle is the Organic Rankine cycle(ORC), which is named for its use of an organic, high molecular massfluid, with a liquid-vapor phase change, or boiling point, occurring ata lower temperature than the water-steam phase change. As such, in placeof water and steam of the classical Rankine cycle, the working fluid inthe ORC may be a solvent, such as n-pentane or toluene. The ORC workingfluid allows Rankine cycle heat recovery from lower temperature sourcessuch as biomass combustion, industrial waste heat, geothermal heat,solar ponds, etc. The low-temperature heat may then be converted intouseful work, which may in turn be converted into electricity.

SUMMARY

In general terms, this disclosure is directed to a volumetric energyrecovery system with a three stage expansion system. In one possibleconfiguration and by non-limiting example, the present disclosurerelates to a method of generating mechanical work via a closed-loopRankine cycle, the method comprising: heating a working fluid to atleast a partial vapor state; generating useful work at a first expansionstage by expanding the working fluid as the working fluid passes throughthe first expansion stage; generating useful work at a second expansionstage by expanding the working fluid as the working fluid passes throughthe second expansion stage; generating useful work at a third expansionstage by expanding the working fluid as the working fluid passes throughthe third expansion stage; and condensing the working fluid to a liquidstate.

Another aspect of the disclosure relates to a system used to generatemechanical work via a closed-loop Rankine cycle, the system comprising:a power plant producing a heat stream and having a heat outlet throughwhich the heat stream exits; a heat exchanging device configured totransfer heat from the heat stream to a working fluid steam; a firstvolumetric fluid expansion stage configured to receive the working fluidstream from the first heat exchanger; a second volumetric fluidexpansion stage configured to receive the working fluid stream from thefirst volumetric fluid expansion stage; and a third volumetric fluidexpansion stage configured to receive the working fluid stream from thesecond volumetric fluid expansion stage. Each of the first, second, andthird volumetric fluid expansion stages is configured to generatemechanical work from the working fluid stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of a first exemplary system employing aRankine cycle using a three stage expansion system in accordance withthe principles of the present disclosure.

FIG. 2 is a schematic depiction of a second exemplary system employing aRankine cycle using a three stage expansion system in accordance withthe principles of the present disclosure.

FIG. 3 is a schematic depiction of a third exemplary system employing aRankine cycle using a three stage expansion system in accordance withthe principles of the present disclosure.

FIG. 4 is a flowchart of an exemplary process in a Rankine cycle with athree stage expansion system.

FIG. 5 is a flowchart of another exemplary process in a Rankine cyclewith a three stage expansion system.

FIG. 6 is a flowchart of yet another exemplary process in a Rankinecycle with a three stage expansion system.

FIG. 7 is a side perspective view of an example of a volumetric fluidexpander having features that are examples of aspects in accordance withthe principles of the present disclosure.

FIG. 8 is a cross-sectional side perspective view of the volumetricfluid expander shown in FIG. 7.

FIG. 9 is a cross-sectional side view of an example of a volumetricfluid expander having features that are examples of aspects inaccordance with the principles of the present disclosure.

FIG. 10 is a schematic perspective top view of the volumetric fluidexpander shown in FIG. 9.

FIG. 11 is a schematic showing geometric parameters of the rotors of thevolumetric fluid expanders shown in FIGS. 7 and 9.

FIG. 12 is a schematic showing the rotors of the volumetric fluidexpanders shown in FIGS. 7 and 9.

FIG. 13 is a diagram depicting the Rankine cycle employed by the systemshown in FIGS. 1-6.

DETAILED DESCRIPTION

Various embodiments will be described in detail with reference to thedrawings, wherein like reference numerals represent like parts andassemblies throughout the several views. Reference to variousembodiments does not limit the scope of the present disclosure.Additionally, any examples set forth in this specification are notintended to be limiting and merely set forth some of the many possibleembodiments for the appended claims.

Referring to the drawings, a system is illustrated in which a pluralityof volumetric fluid expansion stages 20 having dual interleaved rotorsextracts energy from a waste heat stream from a power source, which isalso referred to herein as a power plant, that would otherwise bewasted. As described below, the rotors can be configured to be eitherstraight or twisted. The volumetric fluid expansion stage 20 may also bereferred to herein as an expander, expansion device or volumetric energyrecovery device. An energy recovery system can be formed by couplingcomponents with the output of the volumetric fluid expansion stage thattransfers energy back to the power plant directly or indirectly.

Volumetric Energy Recovery System with Three Stage Expansion System

FIG. 1 is a schematic depiction of a system 100 employing a Rankinecycle using a three stage expansion system in accordance with theprinciples of the present disclosure. In this example, the system 100employs a working fluid 12 as the working substance for closed loopcirculation while using the Rankine cycle to generate mechanical work.The working fluid 12 can be of any type suitable for the Rankine cycle.In some examples, the working fluid is ethanol, n-pentane, or toluene.In this document, the working fluid 12 is designated as differentreference numerals, such as 13, 14, 17, 22, 23, 16, 27, 28, 30 and 32,to represent different phases, temperatures and/or pressures or thefluid 12.

In some examples, the system 100 includes an engine 52; a plurality ofheat exchangers 18-1, 18-2, 18-3, and 18-4 (collectively designated as18); a three stage expansion system having a plurality of expansionstages 20-1, 20-2, and 20-3 (collectively designated as 20); a condenser25; and a fluid pump 16.

The engine 52 can be an internal combustion engine that operates oncombustion of a chemical fuel, such as diesel fuel or gasoline, andproduces a great quantity of heat and exhaust gases. In some examples,the engine 52 can include a supercharger or turbocharger 102 to useforced induction.

The plurality of heat exchangers 18 is configured to transfer heat toand from the working fluid 12 passing therein.

The expansion stages 20 are configured to receive the working fluid 12and generate mechanical work. In operation, as the working fluid 12passes through the expansion stage 20, the temperature and pressure ofthe fluid 12 drop. In general, the expansion stages 20 rely upon thepressure of the fluid 12 to rotate an output shaft, thereby creatingmechanical energy. The mechanical energy can be used or stored inseveral ways. For example, the torque created by the expansion stages 20can be used by the engine 52. Each of the expansion stages 20 can returnthe extracted energy back to the engine 52 via an output shaft 38 of thedevice 20 (FIGS. 3-6). In other examples, the mechanical energy can beaccumulated in a load storage device for subsequent release on demand.The mechanical energy can also be used for an electrical generator thatis associated with the system 100 or for a hydraulic pump used by theengine 52, Accordingly, the volumetric fluid expansion stages 20 operateto increase the overall efficiency of the engine 52, to create usefulmechanical work, and to recirculate the fluid 12. Examples of theexpansion stages 20 are discussed below in further detail with referenceto FIGS. 3-6.

The condenser 25 operates to condense the working fluid 12 from itsgaseous state to a liquid state by cooling it.

The fluid pump 16 is configured to pump the working fluid 12 from low tohigh pressure while maintaining the working fluid 12 in its liquidstate.

Referring to FIG. 1, in operation, the engine 52 receives air throughthe turbo 102. The turbo 102 receives atmospheric air 103 at temperatureT1 and a charge air cooler 104 cools the air 103 to air 106 attemperature T2 that is lower than temperature T1. The air 106 attemperature T2 is then delivered to and used by the engine 52 whichthereafter emits exhaust gas 108 at temperature T3 that is higher thantemperature T2.

The exhaust gas 108 at temperature T3 enters a first heat exchanger18-1. The heat exchanger 18-1 utilizes a fluid 14 at temperature T9flowing therein as a cooling fluid. The temperature T9 is lower than thetemperature T3. As discussed below, the fluid 14 is discharged from afirst expansion stage 20-1. The first heat exchanger 18-1 circulates thefluid 14 through its coils, thereby cooling the exhaust gas 108 as itflows past the coils and simultaneously heating the fluid 14 to producea fluid 13 at temperature T11 that is higher than the temperature T9.The heated fluid 13 at the temperature T11 then passes through a secondexpansion stage 20-2.

The second expansion stage 20-2 receives the fluid 13 at the temperatureT11 and discharges a fluid 17 at temperature T12 that is lower than T11.Furthermore, the fluid 17 has a lower pressure than the fluid 12. Whilereducing the temperature and pressure of the fluid 13 into the fluid 17,the second expansion stage 20-2 generates mechanical work that can beused or stored in various ways as discussed above.

The fluid 17, with a lower temperature and pressure than the fluid 13,immediately flows through a third expansion stage 20-3 which is againused to create mechanical energy as it discharges a fluid 21 attemperature T4 that is lower than the temperature T12. Furthermore, thefluid 21 has a lower pressure than the fluid 17. Because the fluid 17has a lower temperature and pressure than the fluid 13 as it enters thethird expansion stage 20-3, the third expansion stage 20-3 cannotproduce as much mechanical energy as the second expansion stage 20-2. Assuch, the fluid 21 exiting the third expansion stage 20-3 has a lowertemperature and pressure than the fluids 17 and 13. In some examples,the fluid 21 exiting the third expansion stage 20-3 has a mixed phasefluid comprising of a mixture of gas and liquid.

The fluid 21 at the temperature T4 then passes through a second heatexchanger 18-2, which is typically referred to as a recuperator, beforeflowing into the condenser 25. The recuperator 18-2 is placed betweenthe third expansion stage 20-3 and the condenser 25 to further reclaimwaste heat from the fluid 21 released from the third expansion stage20-3. A fluid 23 exiting the recuperator 18-2 has temperature T10 thatis lower than the temperature T4.

The fluid 23 is then sent to the condenser 25 that is used to convertthe fluid 23, which, in some examples, can be a mixture of gas andliquid, to a saturated liquid 31 at temperature T5. As shown in theRankine cycle of FIG. 7, the temperature T5 remains substantially thesame at the condenser 25, and thus the temperature T5 is substantiallythe same as T10.

The fluid 31 at the temperature T5 is pumped from low to high pressureby the pump 16. In this process, the temperature T5 of the fluid 31increases, as shown in the Rankine cycle of FIG. 7. Therefore, a fluid27 discharged from the pump 16 has the temperature T13 that is higherthan the temperature T5 of the fluid 31, and flows into the recuperator18-2.

The recuperator 18-2 utilizes the fluid 27 at the temperature T13 totake heat from the fluid 21, which is released from the third expansionstage 20-3. Accordingly, the recuperator 18-2 transfers heat from thefluid 21 at the temperature T4 to the fluid 27 at the temperature T13,thereby producing a fluid 33 at temperature T6 that is higher than thetemperature T13. The fluid 33 flows to a third heat exchanger 18-3.

The third heat exchanger 18-3 transfers heat from the exhaust gas 108,which is released from the first heat exchanger 18-1, to the fluid 33,thereby producing a fluid 35 at temperature T7 that is higher than T6.The fluid 35 thereafter flows to a fourth heat exchanger 18-4.

The fourth heat exchanger 18-4 transfers heat from the exhaust gas 108at the temperature T3, which flows through the fourth heat exchanger18-4 from the engine 52, to the fluid 35 at the temperature T7. As aresult, the fourth heat exchanger 18-4 produces a fluid 36 attemperature T8 that is greater than T7. The exhaust gas 108 issimultaneously cooled to a lower temperature than T3 as it flows throughthe fourth heat exchanger 18-4 and released to the atmosphere.

The fluid 36 at the temperature T8 is received by the first expansionstage 20-1 that discharges a fluid 14 at temperature T9 that is lowerthan T8 and generates mechanical work as described above. The fluid 14has a lower pressure than the fluid 36. The fluid 14 leaving the firstexpansion stage 20-1 at temperature T9 flows directly to the first heatexchanger 18-1 where it is re-heated directly by exhaust gas 108supplied from the engine 52. The entire process is then repeated in acycle as described above.

As illustrated above, the second heat exchanger 18-2, which is alsoreferred to as the recuperator, the third heat exchanger 18-3, and thefourth heat exchanger 18-4 are connected in series. In some examples,the second, third and fourth heat exchangers 18-2, 18-3 and 18-4 arereplaced by one or two heat exchanger devices, which operate the same asthe combination of the first, second and third heat exchangers 18-2,18-3 and 18-4.

FIG. 2 is a schematic depiction of a second exemplary system 100employing a Rankine cycle using a three stage expansion system inaccordance with the principles of the present disclosure. As many of theconcepts and features are similar to the first example shown in FIG. 1,the description for the first example is hereby incorporated byreference for the second example. Where like or similar features orelements are shown, the same reference numbers will be used wherepossible. The following description for the second example will belimited primarily to the differences between the first and secondexamples.

In this example, the system 100 removes the first heat exchanger 18-1.In the first example, the fluid 14 at the temperature T9 is dischargedfrom the first expansion stage 20-1 and enters the first heat exchanger18-1 before flowing into the second expansion stage 20-2. In contrast,in this example, the fluid 14 at the temperature T9 discharged from thefirst expansion stage 20-1 is directly delivered to the second expansionstage 20-2. Furthermore, in the first example, the exhaust gas 108 atthe temperature T3 supplied from the engine 52 passes through the firstheat exchanger 18-1 and the third heat exchanger 18-3 in series.However, in this example, the exhaust gas 108 at the temperature T3flows directly from the engine 52 to the third heat exchanger 18-3.

FIG. 3 is a schematic depiction of a third exemplary system 100employing a Rankine cycle using a three stage expansion system inaccordance with the principles of the present disclosure. As many of theconcepts and features are similar to the second example shown in FIG. 2,the description for the second example is hereby incorporated byreference for the third example. Where like or similar features orelements are shown, the same reference numbers will be used wherepossible. The following description for the third example will belimited primarily to the differences between the second and thirdexamples.

In this example, the recuperator 18-2 is directly connected to both thethird heat exchanger 18-3 and the fourth heat exchanger 18-4 while thethird heat exchanger 18-3 and the fourth heat exchanger 18-4 arearranged in parallel. The system 100 can include a splitter valve 19(also known as a distributor valve), which operates to divide the fluiddischarged from the recuperator 18-2 to flow into both the third heatexchanger 18-3 and the fourth heat exchanger 18-4 at the same time.Therefore, the fluid 33 at the temperature T6 discharged from therecuperator 18-2 is drawn into both the third heat exchanger 18-3 andthe fourth heat exchanger 18-4. The third heat exchanger 18-3 transfersheat from the exhaust gas 108 of the engine 52 to the fluid 33, anddischarges the fluid 29 at temperature T14, which is greater than thetemperature T6. The fluid 29 flows directly to the first expansion stage20-1. Similarly, the fourth heat exchanger 18-4 transfers heat from theexhaust gas 108 of the engine 52 to the fluid 33, and discharges thefluid 36, which is then drawn to the first expansion stage 20-1.

Although this example describes that the two heat exchangers 18-3 and18-4 are arranged in parallel through one splitter valve 19, more thantwo heat exchangers can be arranged in parallel through one or moresplitter valves, provided that the heat exchangers discharge the fluid36 that is drawn into the first expansion stage 20-1.

Additional examples are directed to a method of using a three stageexpansion system in a Rankine cycle as described in FIGS. 1 and 2.

FIG. 4 is a flowchart of an exemplary method 300 for circulating theworking fluid 12 in a Rankine cycle with a three stage expansion system.At process 302, the working fluid 12 is heated to at least a partialvapor state. The process can be performed by a heat exchanging device,such as the first heat exchanger 18-1, the second heat exchanger 18-2,the third heat exchanger 18-3, or the fourth heat exchanger 18-4, or anycombination thereof. At process 304, the working fluid 12 passes throughthe first expansion stage 20-1, which expands the working fluid 12 andgenerates useful work from expansion. At process 306, the working fluid12 discharged from the first expansion stage 20-1 passes through thesecond expansion stage 20-2. The second expansion stage 20-2 generatesuseful work by expanding the working fluid 12. The working fluid 12 isthen discharged from the second expansion stage 20-2. At process 308,the working fluid 12 passes the third expansion stage 20-3, whichgenerates useful work by expanding the working fluid 12. At process 310,the working fluid 14, which has been used to generate useful work by thefirst, second and third expansion stages 20-1, 20-2 and 20-3, is thencondensed to a liquid state, and returns to the process 302.

FIG. 5 is a flowchart of an exemplary method 200 for circulating theworking fluid 12 in a Rankine cycle with a three stage expansion system.For example, the process 200 can be performed in the system 100 inaccordance with the second example described above with reference toFIG. 2. In this example, the working fluid 36 at the temperature T8enters the first expansion stage 20-1 (202). At process 202, thepressure and temperature of the working fluid 36 decrease as the workingfluid 36 passes through the first expansion stage 20-1 thatsimultaneously generates mechanical energy, which is also referred toherein as useful work. The first expansion stage 20-1 then dischargesthe working fluid 14 at the temperature T9. The working fluid 14 flowsinto the second expansion stage 20-1 (204). At process 204, the pressureand temperature of the working fluid 14 decrease as the working fluid 14passes through the second expansion stage 20-2 that simultaneouslygenerates mechanical energy. The second expansion stage 20-2 thendischarges the working fluid 17 at the temperature T12. The workingfluid 17 flows into the third expansion stage 20-3 (206). At process206, the pressure and temperature of the working fluid 17 decrease asthe working fluid 17 passes through the third expansion stage 20-3 thatsimultaneously generates mechanical energy. The third expansion stage20-3 then discharges the working fluid 21 at the temperature T4.

The working fluid 21 enters the second heat exchanger or recuperator18-2 (208). At process 208, the temperature of the working fluid 21 isreduced to the temperature T10 by the recuperator 18-2. The workingfluid 23 at the temperature T10 then enters the condenser 25, whichliquidizes the fluid 23 and discharges the working fluid 31 at thetemperature T5 (210). As shown in FIG. 7, the temperature T5 of thefluid 31 is substantially the same as the temperature T10 of the fluid23. At process 212, the working fluid 31 is pumped by the pump 16. Theworking fluid 27 pumped from the pump 16 has the temperature T13 that ishigher than the temperature T5 of the fluid 31, as shown in FIG. 7. Atprocess 214, the working fluid 27 is heated by the recuperator 18-2 tohave increased temperature. The recuperator 18-2 produces the workingfluid 33 at the temperature T6 that is higher than T13. At process 216,the working fluid 33 is further heated by the third heat exchanger 18-3to have increased temperature. The third heat exchanger 18-3 dischargesthe working fluid 35 at the temperature T7 higher than T6. At process218, the working fluid 35 is again heated by the fourth exchanger 18-4to have increased temperature. The fourth heat exchanger 18-4 dischargesthe working fluid 36 at the temperature T8 higher than T7. The workingfluid 36 is fed back into the third expansion stage 18-3 at process 202,as described above.

FIG. 6 is a flowchart of another exemplary method 200 for circulating aworking fluid 12 in a Rankine cycle with a three stage expansion system.For example, the process 200 can be performed in the system 200 inaccordance with the first example described above with reference toFIG. 1. As the method in this example is substantially similar to thefirst example shown in FIG. 8, the description for the first example ishereby incorporated by reference for this example. Where like or similarfeatures or elements are shown, the same reference numbers will be usedwhere possible. The following description will be limited primarily tothe differences between the first and second examples.

In this example, the method 200 further includes a step of increasingthe temperature of the working fluid at the first heat exchanger 18-1between processes 202 and 204 (220). At process 220, the working fluid14, which has passed the first expansion stage 20-1, is drawn into thefirst heat exchanger 18-1 to increase its temperature before enteringthe second expansion stage 20-2. As the working fluid 14 passes throughthe first heat exchanger 18-1, the temperature increases from T9 to T11.Thus, the first heat exchanger 18-1 discharges the working fluid 13 atthe temperature T11, which subsequently flows into the second expansionstage 20-2 for process 204.

Volumetric Fluid Expander

FIGS. 7-12 illustrate an expander used in the system shown in FIGS. 1-3.FIG. 7 is a side perspective view of an example of a volumetric fluidexpander having features that are examples of aspects in accordance withthe principles of the present disclosure. FIG. 8 is a cross-sectionalside perspective view of the volumetric fluid expander shown in FIG. 7.FIG. 9 is a cross-sectional side view of another example of a volumetricfluid expander having features that are examples of aspects inaccordance with the principles of the present disclosure. In general,the volumetric energy recovery device 20 relies upon the kinetic energyand static pressure of the working fluid 12-1 to rotate an output shaft38. Where the device 20 is used in an expansion application, such aswith a Rankine cycle, additional energy is extracted from the workingfluid via fluid expansion. In such instances, the device 20 may bereferred to as an expander or expansion device, as so presented in thefollowing paragraphs. However, it is to be understood that the device 20is not limited to applications where a working fluid is expanded acrossthe device.

The expansion device 20 has a housing 22 with a fluid inlet 24 and afluid outlet 26 through which the working fluid 12-1 undergoes apressure drop to transfer energy to the output shaft 38. The inlet port24 is configured to admit the working fluid 12-1 at a first pressurefrom the heat exchanger 18 (shown in FIGS. 1-3), whereas the outlet port26 is configured to discharge the working fluid 12-2 at a secondpressure lower than the first pressure. The output shaft 38 is driven bysynchronously connected first and second interleaved counter-rotatingrotors 30, 32 which are disposed in a cavity 28 of the housing 22. Eachof the rotors 30, 32 has lobes that are twisted or helically disposedalong the length of the rotors 30, 32. Upon rotation of the rotors 30,32, the lobes at least partially seal the working fluid 12-1 against aninterior side of the housing at which point expansion of the workingfluid 12-1 only occurs to the extent allowed by leakage which representsand inefficiency in the system. In contrast to some expansion devicesthat change the volume of the working fluid when the fluid is sealed,the volume defined between the lobes and the interior side of thehousing 22 of device 20 is constant as the working fluid 12-1 traversesthe length of the rotors 30, 32. Accordingly, the expansion device 20may be referred to as a “volumetric device” as the sealed or partiallysealed working fluid volume does not change.

As additionally shown in FIG. 10, each rotor 30, 32 has four lobes,30-1, 30-2, 30-3, and 30-4 in the case of the rotor 30, and 32-1, 32-2,32-3, and 32-4 in the case of the rotor 32. Although four lobes areshown for each rotor 30 and 32, each of the two rotors may have anynumber of lobes that is equal to or greater than two. Additionally, thenumber of lobes is the same for both rotors 30 and 32. This is incontrast to the construction of typical rotary screw devices and othersimilarly configured rotating equipment which have a dissimilar numberof lobes (e.g. a male rotor with “n” lobes and a female rotor with “n+1”lobes). Furthermore, one of the distinguishing features of the expansiondevice 20 is that the rotors 30 and 32 are identical, wherein the rotors30, 32 are oppositely arranged so that, as viewed from one axial end,the lobes of one rotor are twisted clockwise while the lobes of themeshing rotor are twisted counter-clockwise. Accordingly, when one lobeof the rotor 30, such as the lobe 30-1 is leading with respect to theinlet port 24, a lobe of the rotor 32, such as the lobe 30-2, istrailing with respect to the inlet port 24, and, therefore with respectto a stream of the high-pressure working fluid 12-1.

As shown, the first and second rotors 30 and 32 are fixed to respectiverotor shafts, the first rotor being fixed to an output shaft 38 and thesecond rotor being fixed to a shaft 40. Each of the rotor shafts 38, 40is mounted for rotation on a set of bearings (not shown) about an axisX1, X2, respectively. It is noted that axes X1 and X2 are generallyparallel to each other. The first and second rotors 30 and 32 areinterleaved and continuously meshed for unitary rotation with eachother.

The first and second rotors 30 and 32 are interleaved and continuouslymeshed for unitary rotation with each other. With renewed reference toFIG. 9, the expander 20 also includes meshed timing gears 42 and 44,wherein the timing gear 42 is fixed for rotation with the rotor 30,while the timing gear 44 is fixed for rotation with the rotor 32. Thetiming gears 42, 44 are also configured to maintain the relativeposition of the rotors 30, 32 such that contact between the rotors isentirely prevented between the rotors 30, 32 which could cause extensivedamage to the rotors 30, 32. Rather, a close tolerance between therotors 30, 32 is maintained during rotation by the timing gears 42, 44.As the rotors 30, 32 are non-contacting, a lubricant in the fluid 12 isnot required for operation of the expansion device 20, in contrast totypical rotary screw devices and other similarly configured rotatingequipment having rotor lobes that contact each other.

The output shaft 38 is rotated by the working fluid 12 as the workingfluid undergoes expansion from the higher first pressure working fluid12-1 to the lower second pressure working fluid 12-2. As mayadditionally be seen in both FIGS. 9 and 10, the output shaft 38 extendsbeyond the boundary of the housing 22. Accordingly, the output shaft 38is configured to capture the work or power generated by the expander 20during the expansion of the working fluid 12 that takes place in therotor cavity 28 between the inlet port 24 and the outlet port 26 andtransfer such work as output torque from the expander 20. Although theoutput shaft 38 is shown as being operatively connected to the firstrotor 30, in the alternative the output shaft 38 may be operativelyconnected to the second rotor 32. The output shaft 38 can be coupled tothe engine 52 such that the energy from the exhaust can be recaptured.

In one aspect of the geometry of the expander 20, each of the rotorlobes 30-1 to 30-4 and 32-1 to 32-4 has a lobe geometry in which thetwist of each of the first and second rotors 30 and 32 is constant alongtheir substantially matching length 34. As shown schematically at FIG.11, one parameter of the lobe geometry is the helix angle HA. By way ofdefinition, it should be understood that references hereinafter to“helix angle” of the rotor lobes is meant to refer to the helix angle atthe pitch diameter PD (or pitch circle) of the rotors 30 and 32. Theterm pitch diameter and its identification are well understood to thoseskilled in the gear and rotor art and will not be further discussedherein. As used herein, the helix angle HA can be calculated as follows:Helix Angle (HA)=(180/.pi.*arctan (PD/Lead)), wherein: PD=pitch diameterof the rotor lobes; and Lead=the lobe length required for the lobe tocomplete 360 degrees of twist. It is noted that the Lead is a functionof the twist angle and the length L1, L2 of the lobes 30, 32,respectively. The twist angle is known to those skilled in the art to bethe angular displacement of the lobe, in degrees, which occurs in“traveling” the length of the lobe from the rearward end of the rotor tothe forward end of the rotor. As shown, the twist angle is about 120degrees, although the twist angle may be fewer or more degrees, such as160 degrees.

In another aspect of the expander geometry, the inlet port 24 includesan inlet angle 24-1, as can be seen schematically at FIG. 9. In oneexample, the inlet angle 24-1 is defined as the general or average angleof an inner surface 24 a of the inlet port 24, for example an anteriorinner surface. In one example, the inlet angle 24-1 is defined as theangle of the general centerline of the inlet port 24, for example asshown at FIG. 9. In one example, the inlet angle 24-1 is defined as thegeneral resulting direction of the working fluid 12-1 entering therotors 30, 32 due to contact with the anterior inner surface 24 a, ascan be seen at FIG. 9. As shown, the inlet angle 24-1 is neitherperpendicular nor parallel to the rotational axes X1, X2 of the rotors30, 32. Accordingly, the anterior inner surface 24 a of the inlet port24 causes a substantial portion of the working fluid 12-1 to be shapedin a direction that is at an oblique angle with respect to therotational axes X1, X2 of the rotors 30, 32, and thus generally parallelto the inlet angle 24-1.

Furthermore, and as shown in FIG. 9, the inlet port 24 may be shapedsuch that the working fluid 12-1 is directed to the first axial ends 30a, 32 a of the rotors 30, 32 and directed to the rotor lobe leading andtrailing surfaces (discussed below) from a lateral direction. However,it is to be understood that the inlet angle 24-1 may be generallyparallel or generally perpendicular to axes X1, X2, although anefficiency loss may be anticipated for certain rotor configurations.Furthermore, it is noted that the inlet port 24 may be shaped to narrowtowards the inlet opening 24 b, as shown in FIG. 9.

Referring to FIG. 12, it can be seen that the inlet port 24 has a widthW that is slightly less than the combined diameter distance of therotors 30, 32. The combined rotor diameter is equal to the distancebetween the axes X1 and X2 plus the twice the distance from thecenterline axis X1 or X2 to the tip of the respective lobe. In someexamples, width W is the same as or more than the combined rotordiameter.

In another aspect of the expander geometry, the outlet port 26 includesan outlet angle 26-1, as can be seen schematically at FIG. 9. In oneexample, the outlet angle 26-1 is defined as the general or averageangle of an inner surface 26 a of the outlet port 26. In one example,the outlet angle 26-1 is defined as the angle of the general centerlineof the outlet port 26, for example as shown at FIG. 9. In one example,the outlet angle 26-1 is defined as the general resulting direction ofthe working fluid 12-2 leaving the rotors 30, 32 due to contact with theinner surface 26 a, as can be seen at FIG. 9. As shown, the outlet angle26-1 is neither perpendicular nor parallel to the rotational axes X1, X2of the rotors 30, 32. Accordingly, the inner surface 26 a of the outletport 26 receives the leaving working fluid 12-2 from the rotors 30, 32at an oblique angle which can reduce backpressure at the outlet port 26.In one example, the inlet angle 24-1 and the outlet angle 26-1 aregenerally equal or parallel, as shown in FIG. 9. In one example, theinlet angle 24-1 and the outlet angle 26-1 are oblique with respect toeach other. It is to be understood that the outlet angle 26-1 may begenerally perpendicular to axes X1, X2, although an efficiency loss maybe anticipated for certain rotor configurations. It is further notedthat the outlet angle 26-1 may be perpendicular to the axes X1, X2. Asconfigured, the orientation and size of the outlet port 26-1 areestablished such that the leaving working fluid 12-2 can evacuate eachrotor cavity 28 as easily and rapidly as possible so that backpressureis reduced as much as possible. The output power of the shaft 38 ismaximized to the extent that backpressure caused by the outlet can beminimized such that the working fluid can be rapidly discharged into thelower pressure working fluid at the condenser.

The efficiency of the expander 20 can be optimized by coordinating thegeometry of the inlet angle 24-1 and the geometry of the rotors 30, 32.For example, the helix angle HA of the rotors 30, 32 and the inlet angle24-1 can be configured together in a complementary fashion. Because theinlet port 24 introduces the working fluid 12-1 to both the leading andtrailing faces of each rotor 30, 32, the working fluid 12-1 performsboth positive and negative work on the expander 20.

To illustrate, FIG. 10 shows that lobes 30-1, 30-4, 32-1, and 32-2 areeach exposed to the working fluid 12-1 through the inlet port opening 24b. Each of the lobes has a leading surface and a trailing surface, bothof which are exposed to the working fluid at various points of rotationof the associated rotor. The leading surface is the side of the lobethat is forward most as the rotor is rotating in a direction R1, R2while the trailing surface is the side of the lobe opposite the leadingsurface. For example, rotor 30 rotates in direction R1 thereby resultingin side 30-1 a as being the leading surface of lobe 30-1 and side 30-1 bbeing the trailing surface. As rotor 32 rotates in a direction R2 whichis opposite direction R1, the leading and trailing surfaces are mirroredsuch that side 32-2 a is the leading surface of lobe 32-2 while side32-2 b is the trailing surface.

In generalized terms, the working fluid 12-1 impinges on the trailingsurfaces of the lobes as they pass through the inlet port opening 24 band positive work is performed on each rotor 30, 32. By use of the termpositive work, it is meant that the working fluid 12-1 causes the rotorsto rotate in the desired direction: direction R1 for rotor 30 anddirection R2 for rotor 32. As shown, working fluid 12-1 will operate toimpart positive work on the trailing surface 32-2 b of rotor 32-2, forexample on surface portion 47. The working fluid 12-1 is also impartingpositive work on the trailing surface 30-4 b of rotor 30-1, for exampleof surface portion 46. However, the working fluid 12-1 also impinges onthe leading surfaces of the lobes, for example surfaces 30-1 and 32-1,as they pass through the inlet port opening 24 b thereby causingnegative work to be performed on each rotor 30, 32. By use of the termnegative work, it is meant that the working fluid 12-1 causes the rotorsto rotate opposite to the desired direction, R1, R2.

Accordingly, it is desirable to shape and orient the rotors 30, 32 andto shape and orient the inlet port 24 such that as much of the workingfluid 12-1 as possible impinges on the trailing surfaces of the lobeswith as little of the working fluid 12-1 impinging on the on the leadinglobes such that the highest net positive work can be performed by theexpander 20.

One advantageous configuration for optimizing the efficiency and netpositive work of the expander 20 is a rotor lobe helix angle HA of about35 degrees and an inlet angle 24-1 of about 30 degrees. Such aconfiguration operates to maximize the impingement area of the trailingsurfaces on the lobes while minimizing the impingement area of theleading surfaces of the lobes. In one example, the helix angle isbetween about 25 degrees and about 40 degrees. In one example, the inletangle 24-1 is set to be within (plus or minus) 15 degrees of the helixangle. In one example, the helix angle is between about 25 degrees andabout 40 degrees. In one example, the inlet angle 24-1 is set to bewithin (plus or minus) 15 degrees of the helix angle HA. In one example,the inlet angle is within (plus or minus) 10 degrees of the helix angle.In one example, the inlet angle 24-1 is set to be within (plus or minus)5 degrees of the helix angle HA. In one example, the inlet angle 24-1 isset to be within (plus or minus) fifteen percent of the helix angle HAwhile in one example, the inlet angle 24-1 is within ten percent of thehelix angle. Other inlet angle and helix angle values are possiblewithout departing from the concepts presented herein. However, it hasbeen found that where the values for the inlet angle and the helix angleare not sufficiently close, a significant drop in efficiency (e.g.10-15% drop) can occur.

Rankine Cycle Operation

FIG. 13 shows a diagram 48 depicting a representative Rankine cycleapplicable to the system 100, as described with respect to FIGS. 1-6.The diagram 48 depicts different stages of the Rankine cycle showingtemperature in Celsius plotted against entropy “S”, wherein entropy isdefined as energy in kilojoules divided by temperature in Kelvin andfurther divided by a kilogram of mass (kJ/kg*K). The Rankine cycle shownin FIG. 7 is specifically a closed-loop Organic Rankine Cycle (ORC) thatmay use an organic, high molecular mass working fluid, with aliquid-vapor phase change, or boiling point, occurring at a lowertemperature than the water-steam phase change of the classical Rankinecycle. Accordingly, in the system 100, the working fluid 12 may be asolvent, such as ethanol, n-pentane or toluene.

In the diagram 48 of FIG. 13, the term “{dot over (Q)}” represents theheat flow to or from the system 100, and is typically expressed inenergy per unit time. The term “{dot over (W)}” represents mechanicalpower consumed by or provided to the system 100, and is also typicallyexpressed in energy per unit time. As may be additionally seen from FIG.13, there are four distinct processes or stages 48-1, 48-2, 48-3, and48-4 in the ORC. During stage 48-1, the working fluid 12 in the form ofa wet vapor enters and passes through the condenser 25, in which theworking fluid is condensed at a constant temperature to become asaturated liquid. Following stage 48-1, the working fluid 12 is pumpedfrom low to high pressure by the pump 16 during the stage 48-2. Duringstage 48-2, the working fluid 12 is in a liquid state.

From stage 48-2 the working fluid is transferred to stage 48-3. Duringstage 48-3, the pressurized working fluid 12 enters and passes throughthe heat exchanger 18 where it is heated at constant pressure by anexternal heat source to become a two-phase fluid, i.e., liquid togetherwith vapor. From stage 48-3 the working fluid 12 is transferred to stage48-4. During stage 48-4, the working fluid 12 in the form of thetwo-phase fluid expands through the expander 20, generating useful workor power. The expansion of the partially vaporized working fluid 12through the expander 20 decreases the temperature and pressure of thetwo-phase fluid, such that some additional condensation of the two-phaseworking fluid 12 may occur. Following stage 48-4, the working fluid 12is returned to the condenser 25 at stage 48-1, at which point the cycleis then complete and will typically restart.

Typically a Rankine cycle employs a turbine configured to expand theworking fluid during the stage 48-4. In such cases, a practical Rankinecycle additionally requires a superheat boiler to take the working fluidinto superheated range in order to remove or evaporate all liquidtherefrom. Such an additional superheating process is generally requiredso that any liquid remaining within the working fluid will not collectat the turbine causing corrosion, pitting, and eventual failure of theturbine blades. As shown, the ORC of FIG. 13 is characterized by theabsence of such a superheat boiler and the attendant superheatingprocess needed to evaporate all liquid from the working fluid. Thepreceding omission is permitted by the fact that the expander 20 isconfigured as a twin interleafed rotor device which is not detrimentallyimpacted by the presence of a liquid in the working fluid 12.Furthermore, the expander 20 benefits from the presence of such aliquid, primarily because the remaining liquid tends to enhance theoperational efficiency of the expander by sealing clearances between thefirst and second rotors 30, 32, and between the rotors and the housing22. Accordingly, when useful work is generated by the expander 20 in thesystem 100, the working fluid 12 within the expander is present in twophases, i.e., as a liquid-vapor, such that conversion efficiency of theORC is increased. However, it is to be understood that the recoverydevice 20 can be used in configurations involving a superheated gas.

Additionally, a smaller size expander may be used in the system 100 toachieve the required work output. The efficiency will never be above theCarnot efficiency of 63% because that is the maximum Caarnot efficiencyeff=1−Tcold/Thot. The working fluid will likely be ethanol which has amax temp of 350 c before it starts to break down. The expanderefficiency will be less than the peak efficiency of a turbo but theefficiency islands are considerably larger over a greater flow rangethen than the turbo expander so an overall efficiency for a cycle islarger.

The various examples described above are provided by way of illustrationonly and should not be construed to limit the claims attached hereto.Those skilled in the art will readily recognize various modificationsand changes that may be made without following the example examples andapplications illustrated and described herein, and without departingfrom the true spirit and scope of the following claims.

What is claimed is:
 1. A method of generating mechanical work via aclosed-loop Rankine cycle, the method comprising: heating a workingfluid to at least a partial vapor state; generating useful work at afirst expansion stage by expanding the working fluid as the workingfluid passes through the first expansion stage; generating useful workat a second expansion stage by expanding the working fluid as theworking fluid passes through the second expansion stage; generatinguseful work at a third expansion stage by expanding the working fluid asthe working fluid passes through the third expansion stage; andcondensing the working fluid to a liquid state.
 2. A method forgenerating mechanical work via a closed-loop Rankine cycle, the methodcomprising: passing the working fluid through a heat exchanging deviceto increase the temperature of the working fluid; passing the workingfluid through a first volumetric fluid expansion stage to decrease thetemperature and pressure of the working fluid and to create a thirdmechanical work; passing a working fluid through a second volumetricfluid expansion stage to decrease a temperature and pressure of theworking fluid and to create a first mechanical work; passing the workingfluid through a third volumetric fluid expansion stage to decrease thetemperature and pressure of the working fluid and to create a secondmechanical work; condensing the working fluid; and returning the workingfluid to the first volumetric fluid expansion stage.
 3. The method ofclaim 2, wherein the step of passing the working fluid through the heatexchanging device comprises: receiving, by the heat exchanging device, aheat stream from a power plant; and transferring, by the heat exchangingdevice, heat from the heat stream to the working fluid.
 4. The method ofclaim 2, wherein the step of passing the working fluid through the heatexchanging device comprises providing a first heat exchanger arrangedbetween the first volumetric fluid expansion stage and the secondvolumetric fluid expansion stage, the method further comprising: passingthe working fluid through the first heat exchanger to increase thetemperature of the working fluid, wherein the first heat exchanger isconfigured to receive a heat stream from a power plant and transfer heatfrom the heat stream to the working fluid.
 5. The method of claim 4,further comprising, after condensing the working fluid, passing theworking fluid through a second heat exchanger to increase thetemperature of the working fluid.
 6. The method of claim 5, wherein thestep of passing the working fluid through the heat exchanging devicecomprises providing a third heat exchanger arranged downstream of thesecond heat exchanger to receive the working fluid from the second heatexchanger, the method further comprising: passing the working fluidthrough the third heat exchanger to increase the temperature of theworking fluid, wherein the third heat exchanger is configured to receivea heat stream from a power plant and transfer heat from the heat streamto the working fluid.
 7. The method of claim 5, wherein the step ofpassing the working fluid through the heat exchanging device comprisesproviding a third heat exchanger arranged downstream of the second heatexchanger to receive the working fluid from the second heat exchanger,the method further comprising: passing the working fluid through thethird heat exchanger to increase the temperature of the working fluid,wherein the third heat exchanger is configured to receive the heatstream from the first heat exchanger and transfer heat from the heatstream to the working fluid.
 8. The method of claim 6, wherein the stepof passing the working fluid through the heat exchanging devicecomprises providing a fourth heat exchanger arranged downstream of thethird heat exchanger to receive the working fluid from the third heatexchanger, the method further comprising: passing the working fluidthrough the fourth heat exchanger to increase the temperature of theworking fluid, wherein the fourth heat exchanger is configured toreceive a heat stream from a power plant and transfer heat from the heatstream to the working fluid.
 9. A system used to generate mechanicalwork via a closed-loop Rankine cycle, the system comprising: a powerplant producing a heat stream and having a heat outlet through which theheat stream exits; a heat exchanging device configured to transfer heatfrom the heat stream to a working fluid steam; a first volumetric fluidexpansion stage configured to receive the working fluid stream from theheat exchanging device; a second volumetric fluid expansion stageconfigured to receive the working fluid stream from the first volumetricfluid expansion stage; and a third volumetric fluid expansion stageconfigured to receive the working fluid stream from the secondvolumetric fluid expansion stage; wherein each of the first, second, andthird volumetric fluid expansion stages is configured to generatemechanical work from the working fluid stream.
 10. The system of claim9, further comprising a condenser configured to receive the workingfluid from the third volumetric fluid expansion stage and condense theworking fluid.
 11. The system of claim 10, further comprising a pumpconfigured to receive the working fluid from the condenser and pump theworking fluid in the cycle.
 12. The system of claim 10, wherein the heatexchanging device includes a first heat exchanger configured to receivethe heat stream from the power plant, receive the working fluid from thefirst volumetric fluid expansion stage, transfer heat from the heatstream to the working fluid steam, and provide the working fluid streamto the second volumetric fluid expansion stage.
 13. The system of claim10, wherein the heat exchanging device includes a second heat exchangerconfigured to receive the working fluid discharged from the secondvolumetric fluid expansion stage, wherein the working fluid exiting thesecond heat exchanger flows into the condenser, the second heatexchanger further configured to receive the working fluid dischargedfrom the condenser and transfer heat from the working fluid dischargedfrom the third volumetric fluid expansion stage to the working fluiddischarged from the condenser.
 14. The system of claim 13, wherein theheat exchanging device includes a third heat exchanger configured toreceive the heat stream from the first heat exchanger and the workingfluid from the second heat exchanger, the third heat exchangerconfigured to transfer heat from the heat stream to the working fluiddischarged from the second heat exchanger.
 15. The system of claim 14,wherein the heat exchanging device includes a fourth heat exchangerconfigured to receive the heat stream from the power plant and theworking fluid from the third heat exchanger, the fourth heat exchangerconfigured to transfer heat from the heat stream to the working fluiddischarged from the third heat exchanger.
 16. The system of claim 15,wherein the first volumetric fluid expansion stage is arranged betweenthe fourth heat exchanger and the first heat exchanger and configured toreceive the working fluid discharged from the fourth heat exchanger anddischarge the working fluid to the first heat exchanger.